The present invention relates to a method for checking a value document of a specified value document type for the presence of at least one irregularity of at least one specified type, preferably at least one adhesion, preferably an adhesive strip, and/or at least one crease and/or a material removal, and means for carrying out the method.
In this context, value documents are understood to mean sheet-shaped objects, which represent for example a monetary value or an authorization and thus shall not be manufacturable at will by unauthorized persons. Hence, they have features that are not easily produced, in particular copied, whose presence is an indication of authenticity, i.e. of production by an authorized body. Important examples of such value documents are coupons, vouchers, checks and in particular bank notes.
Such value documents may have irregularities or defects which may make them appear unsuitable for a future usage or even are to be regarded as a, perhaps only weak, suspicion on the presence of a forgery.
An example of lacking suitability for a future usage can be, for example, the presence of an adhesive strip on a value document, in particular a bank note. Such adhesive strips are often employed to repair tears in the respective value document. The damaged value document should hence be withdrawn from circulation, if possible. Another example may be that, in polymer bank notes, regions of an opaque layer applied on the polymer substrate before or on printing chip off in the course of time, so that a material removal is the result.
However, adhesive strips may also be used to connect parts of different value documents with each other to produce a forgery.
For recognizing adhesive strips, a value document may be examined, for example, by means of ultrasound transmission. With the aid of the measuring data there can then be checked whether an indication of the presence of an adhesive strip can be recognized.
A further form of irregularities which may impair the bank notes' fitness for circulation are creases and wrinkles. In U.S. Pat. No. 6,595,060B2 there is described, for example, a method for the determination of structural inhomogeneities in sheet material, in particular due to wrinkles or tears, in which ultrasound is employed. In the following, creases are understood to also be wrinkles.
However, such known methods suffer from the ultrasound transmission data being very noisy, which is due to the kind of measurement and the properties of certain types of value documents, and the recognition of adhesive strips or creases or material removals thus being possibly difficult. Likewise, the local resolution is substantially lower than with optical examinations.